Add bio traversal functionality. This is a prereq for doing ide
multiwrites safely and sanely. Patch was originally done by Suparna,
Bartlomiej picked it up and changed the design somewhat. From Bart:

Main idea is now reversed - instead of introducing rq->hard_bio as
pointer for bio to be completed and using rq->bio as pointer for bio
to be submitted, rq->cbio is introduced for submissions and rq->bio
is used for completions

This minimizes changes to block layer and assures that all existing
block users are not affected by this patch.